In organic chemistry and biochemistry, a side chain is a substituent that is attached to a core part of the molecule called the "main chain" or backbone chain. The side chain is a hydrocarbon branching element of a molecule that is attached to a larger hydrocarbon backbone. It is one factor in determining a molecule's properties and reactivity.
Conventions
The placeholder
R is often used as a generic placeholder for
alkyl (saturated hydrocarbon) group side chains in structural formulae. To indicate other non-carbon groups in structure diagrams,
X,
Y, or
Z are often used.
History
The
R symbol was introduced by 19th-century French chemist Charles Frédéric Gerhardt, who advocated its adoption on the grounds that it would be widely recognizable and intelligible given its correspondence in multiple European languages to the initial letter of "root" or "residue": French
racine ("root") and
résidu ("residue"), these terms' respective English translations along with
radical (itself derived from Latin
radix below), Latin
radix ("root") and
residuum ("residue"), and German
Rest ("remnant" and, in the context of chemistry, both "residue" and "radical").
[Jensen W.B., Journal of Chemical Education 87, 360 (2010)]
Usage
Organic chemistry
In
polymer science, the side chain of an
or
offshoot extends from the
backbone chain of a polymer. Side chains have noteworthy influence on a polymer's properties, mainly its crystallinity and
density. An oligomeric branch may be termed a short-chain branch, and a polymeric branch may be termed a long-chain branch.
are different from side chains; they are neither oligomeric nor polymeric.
Biochemistry
In
, which are composed of
amino acid residues, the side chains are attached to the
alpha-carbon atoms of the
amide backbone. The side chain connected to the alpha-carbon is specific for each amino acid and is responsible for determining
ion and polarity of the amino acid. The amino acid side chains are also responsible for many of the interactions that lead to proper
protein folding and function.
Amino acids with similar polarity are usually attracted to each other, while nonpolar and polar side chains usually repel each other. Nonpolar/polar interactions can still play an important part in stabilizing the secondary structure due to the relatively large amount of them occurring throughout the protein.
Spatial positions of side-chain atoms can be predicted based on protein backbone geometry using computational tools for side-chain reconstruction.
See also
